1. Field of the Invention
The present invention relates to a pellicle for lithography, that is, a pellicle for lithography used as a dust guard in the production of semiconductor devices such as LSI and VLSI or liquid crystal display panels, in particular, a pellicle for lithography used for laser beam light exposure used in light exposure which requires high resolution. Furthermore, the present invention relates to a light exposure method and light exposure apparatus for lithography utilizing a pellicle as well as a method for relieving warpage of a pellicle membrane.
2. Related Art
In the production of semiconductor devices such as LSI and VLSI or liquid crystal display panels, patterning step is conventionally performed by irradiating light on original substrates for semiconductor devices (semiconductor wafers) or original substrates for liquid crystal panels. In this case, dust particles adhered to a light exposure original (photomask) to be used may cause problems. That is, a transferred pattern may be deformed or edge lines of the pattern may be rugged without smoothness, because the dust particles may absorb or reflect the light. Therefore, dimension, appearance, quality and so forth of the pattern may be degraded, and thus performance and production yield of semiconductor devices, liquid crystal display panels and the like may be degraded.
For this reason, these operations are usually performed in a clean room. However, because it is difficult to keep a light exposure original always clean even in such a clean room, a pellicle well transmitting light is applied to a surface of the light exposure original for dust particle guard. That is, a pellicle is adhered to a mask (in the present invention, a light exposure original and a support substrate for the light exposure original are collectively referred to as xe2x80x9cphotomaskxe2x80x9d or xe2x80x9cmaskxe2x80x9d) in order to prevent adhesion of dust particles on the light exposure original used for the light irradiation.
In this case, if a pellicle is applied to a mask, dust particles do not adhere directly on the light exposure original, but on the pellicle membrane. Therefore, if the light is focused on the pattern of the light exposure original during the lithography process, the dust particles on the pellicle are defocused, and hence it provides an advantage that the presence of such dust particles does not influence on the pattern transfer.
As shown in FIG. 1, such a pellicle generally comprises a thin pellicle membrane 1, frame 3, membrane adhesive 2 that adheres the pellicle membrane 1 to the frame 3 and reticle adhesive 4 that adheres the pellicle 11 to a mask 5.
Such a pellicle 11 as shown in FIG. 1 has a structure that a transparent pellicle membrane 1 made of a material well transmitting light such as nitrocellulose, cellulose acetate and fluoropolymer is adhered to one surface of a pellicle frame made of aluminum, stainless steel, polyethylene or the like. In the structure, the pellicle is adhered to the pellicle frame by applying a solvent well dissolving the pellicle membrane 1 on one side surface of the pellicle frame 3 and dried in air (refer to Japanese Patent Laid-open Publication No. 58-219023), or with a membrane adhesive 2 such as acrylic resin, epoxy resin or fluoropolymer (refer to U.S. Pat. No. 4,861,402, Japanese Patent Publication No. 63-27707 and Japanese Patent Laid-open Publication No. 7-168345). Further, the pellicle has an adhesive layer (reticle adhesive) 4 comprising a polybutene resin, polyvinyl acetate resin, acrylate resin, silicone resin or the like and a release layer (separator, not shown) that protects the adhesive layer, which are adhered on the other side. As the pellicle membrane 1, a pellicle thick plate 7 (FIG. 5) such as a glass thick plate may be used. At the time of lithography, the pellicle 11 is adhered beforehand to a mask 5, before it is mounted on a light exposure apparatus. This adhesion to the mask 5 is attained by peeling the release layer and pressing the pellicle 11 to the mask 5 by applying a load. Therefore, the adhesive layer 4 of the pellicle 11 must have a certain thickness and elasticity.
The mask 5 adhered with the pellicle 11 is held and transferred by a mask holding part 8 (FIG. 2) of a light exposure apparatus, and then exposed to light at a predetermined position on the light exposure apparatus.
As for the adhesion of a conventional pellicle 11 and mask 5, there may be a case where a surface of pellicle membrane 1 (henceforth referred to as xe2x80x9cpellicle membrane surfacexe2x80x9d) is not parallel to a surface of a light exposure original of mask 5 (henceforth referred to as xe2x80x9cmask surfacexe2x80x9d).
As the cause of the above phenomenon, there can be mentioned uneven thickness of the adhesive layer provided on the lower surface of the frame 3, uneven pressing upon adhesion, dimensional error in processing of the frame 3 itself and so forth.
If the pellicle membrane surface and the mask surface are not parallel, light passing through the pellicle is distorted and such distortion is transferred on the light-exposed original to be irradiated with the light. Therefore, distortion of circuit pattern is caused on the light-exposed original. This distortion causes a deviation of the pattern line width of the circuit pattern on the light-exposed original, and leads to decrease of yield. Under the recent situation that a wavelength of the light to be used becomes shorter and is shifted to ultraviolet light due to use of finer mask pattern, it becomes increasingly important to secure the parallelism of the pellicle membrane surface and the mask surface.
Further, when a thick plate composed of glass or the like is used as a pellicle thick membrane 7, since it is a thick plate, the interference effect by a thin membrane is eliminated and the reflection at the surface of the pellicle thick plate becomes strong. Moreover, since the pellicle thick plate optically serves as a lens, it becomes more important to secure parallelism of the pellicle membrane surface and the mask surface.
However, in conventional light exposure apparatuses, no attention has been paid at all for adjustment of the parallelism between the pellicle membrane surface and the mask surface.
Moreover, as described above, the resolution of lithography has gradually become higher in recent years, and to realize such resolution, light of a shorter wavelength has gradually come to be used as a light source. Specifically, ultraviolet lights (g-line with wavelength of 436 nm and i-line with wavelength of 365 nm) are currently replaced with far-ultraviolet lights (KrF excimer laser, wavelength of 248 nm), and vacuum ultraviolet lights (ArF excimer laser, wavelength of 193 nm) are ready to be put into practical use in near future. Furthermore, in the lithography using F2 (fluorine) excimer laser (158 nm) in order to realize higher resolution, use of glass plates composed of an inorganic compound or the like as the pellicle membrane is examined.
As the inorganic compound that can be used as the pellicle membrane, substances showing high transmittance in the vacuum ultraviolet region can be used. Specifically, fluorine-doped quartz glass, magnesium fluoride, calcium fluoride, lithium fluoride, aluminum oxide and so forth can be mentioned, and vitreous transparent plates of these are used.
When these inorganic compounds are used as the pellicle membrane, the membrane is desired to have a thickness of 0.1 mm or more in view of strength. However, in such a case, there may be caused a phenomenon that the inorganic compound plate (hereafter it may be referred to as xe2x80x9cglass platexe2x80x9d) may be deflected by its own weight. This deflection may cause deviation of light path for light exposure at the pellicle membrane surface, and thus adversely affect the light exposure.
The present invention was accomplished in order to solve these problems, and its object is to improve resolution in the lithography using a pellicle. That is, one of the specific objects of the present invention is to enable adjustment of parallelism between a pellicle membrane surface and a mask surface upon the light exposure in the lithography utilizing a pellicle.
Furthermore, another specific object of the present invention is to provide a pellicle of high performance comprising a pellicle membrane and a pellicle frame, which does not cause deviation of light path for light exposure due to deflection of a glass plate caused by its own weight and thus does not adversely affect resolution of the lithography, when a glass plate is used as the pellicle membrane in order to use vacuum ultraviolet light of a short wavelength as a light source.
In order to achieve the aforementioned objects, according to the first aspect of the present invention, a method for light exposure of the present invention in lithography using a pellicle is characterized in that parallelism of a pellicle membrane surface of a pellicle and a mask surface is adjusted and secured during light exposure.
The light exposure apparatus of the present invention is characterized in that the light exposure apparatus is provided with a mask holding part and a pellicle holding part or a pellicle supporting part. This pellicle supporting part has a structure that can adjust the pellicle membrane surface to be parallel to the mask surface.
Further, in the pellicle of the present invention used for the lithography, the pellicle membrane consisting of a thick plate can protrude outward from the frame, and the protruding portion may be used as a part to be held that is held by the aforementioned pellicle holding part.
Further, according to the second aspect of the present invention, there is provided a pellicle for lithography, wherein a glass plate is used as a pellicle membrane of the pellicle for lithography, and the glass plate is formed beforehand to have warpage and adhered on a pellicle frame so that a convex surface of the glass plate should become an upper surface.
In the aforementioned structure, the convex warpage is relieved by the own weight of the glass plate, and thus the pellicle can be a pellicle having an extremely flat pellicle membrane without deflection. Therefore, it enables extremely accurate and precise lithography using vacuum ultraviolet light without causing deviation of light path for light exposure during the lithography.
The present invention further provides a pellicle for lithography, wherein a glass plate is used as a pellicle membrane of the pellicle for lithography, and the glass plate is adhered to a preliminarily deformed pellicle frame on which the glass plate is to be placed, so that the glass plate should be given tension by stress obtained by resilience of the pellicle frame.
If a pellicle is constructed as described above, by deforming the pellicle frame inward beforehand, adhering the pellicle membrane on it, and then releasing the distortion of the frame, tension is given to the pellicle membrane by the resilience of the pellicle frame, and thus a pellicle having a the pellicle membrane of excellent flatness can be realized.
The present invention also provides a pellicle for lithography, wherein a glass plate is used as a pellicle membrane of the pellicle for lithography, and a space surrounded by the pellicle comprising the glass plate and a pellicle frame and a photomask is decompressed.
If a space surrounded by the pellicle and the photomask is decompressed when the pellicle is adhered on the photomask as described above, and the pellicle is disposed so that it should be under the photomask, the pellicle can be a pellicle of which warpage due to its own weight is relieved because the pellicle membrane is given a force from below by atmospheric pressure.
Further, the present invention also provides, as a method for relieving warpage of a pellicle membrane of the pellicle for lithography according to the present invention, a method for relieving warpage of a pellicle membrane of a pellicle for lithography utilizing a glass plate as the pellicle membrane, which comprises making the glass plate as the pellicle membrane to have warpage, and adhering the glass plate having warpage to a pellicle frame with an adhesive so that a convex surface of the glass plate should become an upper surface to relieve downward warpage of the glass plate due to its own weight.
In such a manner, the downward warpage of the glass plate due to its own weight is relieved by the convex shape of the glass plate, and thus an extremely flat pellicle membrane with no deflection can be formed. Therefore, deviation of light path for light exposure during the lithography can be obviated, and extremely accurate and precise lithography can be performed.
The present invention also provides, as a method for relieving warpage of a pellicle membrane of the pellicle for lithography according to the present invention, a method for relieving warpage of a pellicle membrane of a pellicle for lithography utilizing a glass plate as the pellicle membrane, which comprises adhering the glass plate as the pellicle membrane with an adhesive to a preliminarily deformed pellicle frame on which the glass plate is to be placed, so that the glass plate should be given tension by stress obtained by resilience of the pellicle frame to relieve downward warpage of the glass plate due to its own weight.
In this method, for example, by deforming the pellicle frame inward beforehand, adhering the pellicle membrane on it, and then releasing the distortion of the frame, tension is given to the pellicle membrane by the resilience of the pellicle frame, and thus the downward warpage of the glass plate caused by its own weight can be relieved. Therefore, an extremely flat pellicle membrane can be formed.
In this case, the method for the deformation and the restoration of the pellicle frame may be a mechanical method or a method utilizing a temperature difference.
Extremely effective are a method of forming inward distortion for each side of the pellicle frame as the mechanical method, a method of shrinking the pellicle frame by cooling it as the method utilizing a temperature difference, and so forth.
The present invention further provides, as a method for relieving warpage of a pellicle membrane of the pellicle for lithography according to the present invention, a method for relieving warpage of a pellicle membrane of a pellicle for lithography utilizing a glass plate as the pellicle membrane, which comprises adhering a pellicle comprising the glass plate as the pellicle membrane and a pellicle frame to a photomask under reduced pressure, and disposing the photomask adhered with the pellicle in such a manner that the pellicle should be under the photomask when they are used, so that downward warpage of the glass plate due to its own weight should be relieved.
If a space surrounded by the pellicle and the photomask is decompressed when the pellicle is adhered to the photomask, and the pellicle is disposed so that it should be under the photomask as described above, warpage of the pellicle membrane due to its own weight is relieved because it is given a force from below by atmospheric pressure, and thus a pellicle having an extremely flat pellicle membrane can be formed.
According to the first aspect of the present invention, the parallelism of the pellicle membrane surface with respect to the mask surface is adjusted and secured by holding the mask with a mask holding part of a light exposure apparatus and holding the pellicle with a pellicle holding part during light exposure. Thus, the parallelism of pellicle membrane surface with respect to the mask surface is enhanced, and light exposure of high quality becomes possible.
Further, according to the second aspect of the present invention, as for a glass plate used as a pellicle membrane when a light source of a shorter wavelength is used, warpage of the glass plate due to its own weight can be relieved. Therefore, there can be provided a highly flat pellicle comprising a pellicle membrane and a pellicle frame of high performance, which does not cause deviation of light path for light exposure due to deflection of a glass plate caused by its own weight, and thus which does not adversely affect the resolution of lithography.